Capacitive Touch Sensor Design Cycles

[Pascal]

Something I found useful and i would like to share with the community in order to have a better view on touch sensor design cycles (since i read this thread here) and i understand there is some confusion.

A capacitive touch sensor design is not an easy task. Designers have to make a lot of decisions and consider many factors. New components find their way into the design along the way, so ensuring that they all work together is key.

 The touch sensor design is split in two parts:

1) the mechanical design of the sensor
2) the controller programming and tuning

The designer chooses a sensor pattern (e.g. Double Diamond),  stack-up materials and a touch sensor controller. Then follows the controller-sensor system testing (with prototyping or simulation), to make sure they are working well together. If they are not compatible, the designer might have to go back to the previous stage and change the sensor pattern or its parameters or tweak the configuration of the controller.
As you can see in the infographic below there can be 3-6 cycles per design till we reach a design that fulfills the requirements.

The more re-designs we do, the more prototypes we need to build. By creating virtual prototypes though and test them, we ensure that only the best design reaches the prototyping stage and is compatible with the controller of our choice. You can read the whole article here.

 

[nctnico]

This seems like scaremongering to buy a product to me. I have done some capacitive touch designs and it is not difficult at all. Got it right the first time every time. Just read the documentation and make sure the capacitance of the pads is within range.

I smell a sock-puppet account.

[coppice]

This seems like scaremongering to buy a product to me. I have done some capacitive touch designs and it is not difficult at all. Got it right the first time every time. Just read the documentation and make sure the capacitance of the pads is within range.

I smell a sock-puppet account.

You must be unique. Getting a cap touch design kinda working is usually pretty easy. Getting it robust is usually pretty hard.

[Pascal]

nctnico yes i work a for touch sensor simulation software company, however my job is not to sell as more to understand, i came across that unanswered thread and i really thought a good idea to start another thread. If the logo at infographic is the problem i can delete it.

I have done some capacitive touch designs and it is not difficult at all. Got it right the first time every time

However i am really curious about what do you mean? What were these designs about? Were for example, capacitive buttons or were there touch interfaces with screen?

We have seen non-electrical engineers that have to solve capacitive touch sensor design "riddles" in their projects. Moreover, more or less the main material used was standard (ITO) however that gradually changes, specially if we talk large touchscreens (50 inches etc) or any bendable/foldable project.

I am keen to hear coppice too. I totally agree and the problem of touch sensor responsiveness becomes even more obvious in the automotive or aviation industry where requirements are quite strict. A touchscreen in a mobile phone is ok to last 2-3 years but in automotive industry? They should last 10+ years. What do you think is needed for a robust capacitive touch sensor design?

[Rerouter]

Reliability is all down to how much time you set aside to check your math,

Is the capacitance in a good part of the dynamic range of the sensor? (What about when coated with water?)
What is the tolerance for the dielectric of your material?
What is the crosstalk between adjacent inputs? (This is one of the big killers)
Is the TX planes surface area suitable?


A derivative decimation for touch events works well for easy stuff, but to really avoid things like rain interference is where it gets harder, however for rain, a correlation filter helps remove most of the issues. When you press 1 button, the nearby buttons will change in value at a ratio, where as a rain drop will generally only hit 1-2., equally a touch press will spike and return to a similar value, while a rain drop will maintain a higher rest value.

This is all basic math, the only things that should make a touchscreen that worked after 1 year, different from one that worked for 10 would be defects in the circuit. or change in material properties, Most of the plastic screen covers put in front of automotive screens actually change in chemistry over time, mostly from cleaning products, less so UV these days. however most of these have there touch detection tolerance high enough that it can work with or without the cover.

[Pascal]

I agree Rerouter, although it seems sunlight is still a problem.

I've been at Tesla forums and i remember the phantom touch problems about Tesla Model 3 were circulating the internet.

According to what i've seen so far there is a speculation that pedestal is quite large and acts as a heat reservoir that causes heat differentials across the touchscreen, leading to phantom touches.

Here a few captures of my phantom touch. It only happens in the afternoon when the car has been in the sun all day. It is worse on hot days

You can check the related thread at Tesla forums here.

However, Tesla is one of those OEMs that they are actually have touch designers in their engineering teams and they were the first that they tried out such a large touchscreen in a car. They even have implemented touchscreens in their spaceships.

Generally these are the 9 things that we have seen so far that affect the cover glass

Cover glasses’ mechanical properties
1. Thickness
2. Weight
3. Scratch resistance
4. Drop resistance

Cover glasses' optical properties
5. Transmittance
6. Anti-glare
7. Anti-haze

Cover glasses’ geometrical properties
8. Finger’s angle and distance relative to the electrodes
9. Signal-to-Noise Ratio (probably No1 in some cases)

[coppice]

According to what i've seen so far there is a speculation that pedestal is quite large and acts as a heat reservoir that causes heat differentials across the touchscreen, leading to phantom touches.

Have you ever encountered a cap touch panel that is completely immune to phantom touches?

[Pascal]

Well there are some suppliers that claim that their cap touch sensor patterns work like almost 90% of the time. Have you encountered such a cap touch panel as well?